Prior Art
U.S. Pat. No. 4,142,583 issued Mar. 6, 1979 describes a shock tool for producing a differential pressure across perforations. The tool is lowered through a tubing string on a wire line and, at a selected level, the wire line connection to the tool is released so that a pressure differential sets a pack off means to close off the cross-section of the tubing string. The fluid in the bore of the tubing string is removed thereby creating a pressure differential across the tool. Next, a sinker bar is dropped from the surface through the tubing to operate a shear pin release whereby the pack off means is released and the perforations are subject to the differential pressure across the tool which provides a back flow on the perforations. Release of the pack off means permits the tool to irretrievably drop to the bottom of the well bore.
Some of the disadvantages of the above system are readily apparent. The tool becomes junk in the well and is not reusable. Also, the exact differential pressure acting on the perforation is imprecise because it depends upon the fluid removal above the tool and is not easily measured or calculated.
U.S. Pat. No. 4,285,402 issued Aug. 25, 1981 also discloses a system for cleaning perforations by differential pressures. In this system, a tool is lowered through a tubing string on a pipe. A manifold at the surface controls this pressure in the pipe and in the tubing string. A packer on the tool is sealed and locked in the tubing. The tool has a bypass passageway normally closed off by a piston which is held in place by a shear pin. FIG. 3 shows a packer variation for seating in a seating nipple. FIG. 4 shows a packer variation and a latching piston mechanism. FIG. 5 shows a landing groove in a tubing string and locking keys for a latching mechanism. In all of the various configurations, a piston is shear pinned in place and when a predetermined differential reaches a force sufficient to shear the shear pin, the piston is released to open a bypass and apply a sudden pressure differential across the perforations. Subsequently, the tool is released by straight up retrieval (FIG. 3) or by pressure control (FIG. 4) or by a retrieving tool (FIG. 6) while the above system is retrievable, its useful life is quite limited because the impact of the suddenly released piston in the tool damages the tool requiring expensive reworking. Also, while the shear pin release can be reasonably calculated it is imprecise as to when the release will occur.